The present invention relates to an anode support system for supplying electric current to a molten salt electrolytic cell and in particular a cell used for producing aluminum.
Aluminum is produced electrolytically from aluminum oxide by dissolving the aluminum oxide in a fluoride melt which is made up for the most part of cryolite. The cathodically deposited aluminum collects under the fluoride melt on the carbon floor of the cell where the surface of the liquid aluminum serves as the cathode. Dipping into the melt are anodes which are secured from above on anode beams. In conventional processes the anodes are made of carbon. As a result of the electrolytic decomposition of the aluminum oxide, oxygen is formed at the carbon anodes and reacts with the carbon to form CO and CO.sub.2. The electrolytic process generally takes place at a temperature of 940.degree.-970.degree. C. In the course of this process the electrolyte is depleted of aluminum oxide. At a concentration in the electrolyte of 1-2 wt.% aluminum oxide the so-called anode effect occurs which results in a stepwise voltage increase from, for example, 4-4.5 V to 30 V and more. At this time at the latest the solid crust of electrolyte formed on the cell must be broken open and the aluminum oxide concentration increased by adding new aluminum oxide (alumina).
Under normal operating conditions the crust on electrolytic cell is usually broken open and fresh alumina fed to the cell at regular intervals even if no anode effect has occurred.
On increasing the current supplied to the cell to a value of 50 kA (kilo ampere) harmful magnetic effects are observed namely a greater upward doming or streaming of the liquid metal in the cell occurs. The reasons for these effects are described in detail in the relevant technical literature, and have led to numerous suggestions of ways to avoid them. The disadvantages arising from the doming and streaming of the metal has also been the subject of many articles.
Both of the above mentioned magnetic effects are however to be differentiated from a further magnetic effect namely the moving wave of metal. This wave of metal runs, depending on the general direction of current flow in the pot line hall, either clockwise or counter-clockwise along the ledge of the cell.
All three magnetic phenomena discussed above have the same root cause namely they are due to an unfavorable distribution of current densities and magnetic induction in the melt.
Publications have been made describing related theories for the doming and streaming of the liquid aluminum. No satisfactory explanation has, however, yet been provided relating current density and induction on the one hand and the creation, persistence and propagation of the metal wave on the other hand. In spite of this, the metal wave rotating right or left, generally along the edge of the bath can be detected, described and followed in the cell.
Wherever the wave is in the cell at any given moment the interpolar distance to the above lying anode becomes smaller. Along with this reduction in the interpolar distance, the resistance in the electrolyte to the direct electric current is also reduced, thereby causing a momentary rise in current at the peak of the wave. As the sum of the currents from all anodes at any given moment corresponds to the direct current value of the cell, the levels of current outwith the region of the metal wave are reduced, in accordance with the interpolar distance, until the wave in the metal has moved further.
The moving wave leads to a change in current level in the individual anodes which varies in time in a sine-wave-like function, whereby however the level of direct current in the anode rod remains constant. The time the wave takes to pass round the cell i.e. the time until it returns to the same anode rod is usually between 30 and 80 seconds.
The reduction in the interpolar distance by the moving wave in the metal brings liquid aluminum, which has already been produced in the process, near the gaseous CO.sub.2 which is formed at the carbon anode. This causes some of the aluminum to be reoxidized to Al.sub.2 O.sub.3, resulting in a lower yield from the cell and correspondingly a lower current efficiency.
One counter-measure here is to increase the interpolar distance at all anodes. This usually reduces the height of the wave and can often even eliminate it altogether. By increasing the interpolar distance, however, the ohmic voltage drop in the electrolyte is raised, and consequently the amount of electrical energy which is consumed is converted to heat instead of producing aluminum. As a result of the lower metal yield the aluminum produced in each unit becomes much more expensive. By simultaneously measuring the current in all the anode rods, by means of standard measuring methods, the position of the metal wave can be readily determined and its movement followed.
The height of the metal wave is some millimeters to some centimeters. In extreme cases it can even cause momentary short circuiting between the cathode and the anode as the interpolar distance is of the same order of magnitude, usually between 4 and 6 cm.
On increasing the interpolar distance both the amplitude of the metal wave and that of the alternating current in the anode rod current decrease. From numerous measurements and observations it has been deduced that the resultant alternating current is due solely to the wave in the metal. Once the wave has been created, as is always the case, the alternating current is responsible for maintaining and propagating the wave.
It is therefore a principal object of the present invention to provide a cell for the electrolysis of fused salts wherein the metal wave is markedly reduced or suppressed without increasing the interpolar distance between the metal wave and the above lying anode.